Setting Up a Modern Express API with TypeScript, Jest, ESLint, Prettier - Quick guide 2025

[Lomanu4]

Building a backend API in Node.js? Setting up the project can be a bit of a slog - and enough to put you off before you even start building that next-gen social media app.

I couldn't help but wonder: where are the quick, modern guides to spinning up a well-structured Express API?

There aren’t many. So I made one.

Let’s get straight to it.

? 1. Initialise your project
npm init -y

This sets up your package.json, which manages your dependencies, scripts, and project metadata. The -y flag accepts all the default values so you can move on quickly.

? 2. Install Express and dev dependencies
npm install express
npm install --save-dev typescript supertest nodemon jest ts-jest ts-node @types/jest @types/supertest @types/express

What these do:

• express: Your HTTP server framework.
• typescript: For strongly typed JS.
• supertest: For testing your API endpoints.
• jest, ts-jest: Testing framework and TypeScript preprocessor.
• nodemon: Restarts the server on changes.
• ts-node: Runs TypeScript files directly.
• @types/...: Adds type support for testing and Express.

? 3. Configure TypeScript
npx tsc --init

Now replace your generated tsconfig.json with:


{
"exclude": [
"./coverage",
"./dist",
"**/*.test.ts",
"jest.config.js",
"eslint.config.mjs"
],
"ts-node": {
"transpileOnly": true,
"files": true
},
"compilerOptions": {
"target": "es2016",
"module": "commonjs",
"rootDir": "./src",
"moduleResolution": "node",
"checkJs": true,
"outDir": "./dist",
"esModuleInterop": true,
"forceConsistentCasingInFileNames": true,
"strict": true,
"noImplicitAny": true,
"skipLibCheck": true
}
}

The linter may freak out, until you create your first .ts file, which we will do now. ;)

? 4. Create your Express app
Here's the directory structure for our app:

mkdir -p src/routes && touch src/app.ts src/app.test.ts src/server.ts src/routes/user.routes.ts src/routes/user.routes.test.ts


src/
├── app.ts
├── app.test.ts
├── server.ts
└── routes/
└── user.routes.ts
└── user.routes.test.ts

src/server.ts


import app from './app';

const PORT: number = 5050;

app.listen(PORT, (): void => {
// eslint-disable-next-line no-console
console.log(`Server is running on ${PORT}...`);
});

src/app.ts


import express, { Application, Request, Response} from 'express';
import { router as userRoutes } from './routes/user.routes';

const app: Application = express();

app.use('/users', userRoutes);

app.use('/', (req: Request, res: Response): void => {
res.json({ message: "Miley, what's good?" });
})

export default app;

src/routes/user.routes.ts


import { Router, Request, Response } from 'express';

const router = Router();

router.get('/', (req: Request, res: Response): void => {
const users = ['Nicki', 'Ariana', 'Lana', 'Miley'];
res.status(200).send(users);
});

export { router };

? 5. Set up testing
npx ts-jest config:init

Replace the generated jest.config.js's contents with:


export const preset = 'ts-jest'
export const testEnvironment = 'node'

This sets up Jest to use the TypeScript compiler.

? 6. Add some tests

src/app.test.ts


import request from 'supertest';
import app from './app';

describe('Test app.ts', () => {
test('Is alive route', async () => {
const res = await request(app).get('/');
expect(res.body).toEqual({ message: "Miley, what's good?" });
});
});

src/routes/user.routes.test.ts


import request from 'supertest';
import app from '../app';

describe('User routes', () => {
test('Get all users', async () => {
const res = await request(app).get('/users');
expect(res.body).toEqual(['Nicki', 'Ariana', 'Lana', 'Miley']);
});
});

7. Update package.json scripts
Update the scripts section:


"scripts": {
"test": "jest --coverage",
"dev": "nodemon ./src/server.ts",
"build": "tsc",
"lint": "eslint src/**/*.ts",
"lint:fix": "eslint . --ext .ts,.tsx,.js,.jsx --fix"
}

(Don't worry about the lint scripts for now, we will set this up in the following step.)

While you're at it, change "type": "commonjs" to "type":"module"
This tells Node.js to treat files as ES Modules.

Give this a go: npm run test

? 8. Set up ESLint + Prettier

npm install -D eslint @eslint/js @types/eslint__js typescript-eslint eslint-plugin-prettier eslint-config-prettier

Now create the ESLint config file touch eslint.config.mjs:


import js from '@eslint/js'
import tseslint from 'typescript-eslint'
import eslintPluginPrettier from 'eslint-plugin-prettier'
import prettier from 'eslint-config-prettier'

export default [
js.configs.recommended,
...tseslint.configs.recommended,
{
files: ['**/*.ts'],
languageOptions: {
parser: tseslint.parser,
parserOptions: {
project: './tsconfig.eslint.json',
sourceType: 'module',
},
},
plugins: {
prettier: eslintPluginPrettier,
},
rules: {
'no-unused-vars': 'error',
'no-undef': 'off',
'prefer-const': 'error',
'no-console': 'warn',
'no-debugger': 'warn',
'prettier/prettier': [
'error',
{
singleQuote: true,
semi: true,
trailingComma: 'all',
printWidth: 100,
tabWidth: 2,
},
],
},
},
prettier,
{
ignores: ['dist/**', 'node_modules/**', 'coverage/**'],
},
]

Let's also add TypeScript config for ESLint

Create a touch tsconfig.eslint.json:


{
"extends": "./tsconfig.json",
"include": ["src/**/*.ts", "src/**/*.tsx"],
"exclude": ["node_modules", "dist"]
}

You now have:

A strongly typed Express API ?

Tests with Jest + Supertest ?

Auto formatting & linting with ESLint + Prettier

You’re all set to build something real - without battling the boilerplate every time.

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I'm János, I write about Software, coding, and technology.
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